The long-term goal is the elucidation, at the atomic level, of phenomena and mechanisms important to the understanding and ultimate modification of the biological occurrence, function and dysfunction of the apatitic portion of dental hard tissue. Various atomic/ionic species present in human tooth enamel (TE) are being studied (1) for their precise structural locations and roles and (2) for the atomic-scale mechanisms by which they affect diffusion, acid dissolution, and the response of TE to various treatments and environments. The structural model for the crystalline portion of TE is constantly being improved toward realism as the locations and roles are further clarified for CO 2 over 3 minus, HPO 2 over 4 minus, H2O, Na ion, Mg 2ion, Sr 2ion Zn 2ion, Pb 2ion, vacancies, and for C1 minus, F minus and oxygen substituting for them. Principal experimental methods are x-ray and neutron diffraction (especially Rietveld analysis), i.r. spectroscopy, deuterizability, EXAFS, and various chemical preparation and analysis methods. Studies are carried out both directly on TE and on related apatite compounds.